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    VA Loading for panel Schedule

    I am reviewing a new panel schedule design that has a three-phase 208V 30A ckt breaker. From my understanding each leg can handle 24A with the 80% rule from the NEC, that is why we installed a 30A breaker. So what should the VA load be on the panel schedule for each phase? A single-phase calculation (at 2880 VA per phase) or three-phase calculation (at 8636 VA per phase) ? I should know this, but having a brain freeze. I think 2880 VA per phase. So any help to unlock my mind would be appreciated!

    #2
    Originally posted by Nardie View Post
    I am reviewing a new panel schedule design that has a three-phase 208V 30A ckt breaker. From my understanding each leg can handle 24A with the 80% rule from the NEC, that is why we installed a 30A breaker. So what should the VA load be on the panel schedule for each phase? A single-phase calculation (at 2880 VA per phase) or three-phase calculation (at 8636 VA per phase) ? I should know this, but having a brain freeze. I think 2880 VA per phase. So any help to unlock my mind would be appreciated!
    Three phase calculation at 2.88-kVA per phase. 8.64-kVA total.


    Sent from my iPhone using Tapatalk

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      #3
      Can you explain to me the "80% rule from the NEC" being referred to?

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        #4
        The 80% rule is the amount of continuous load that the OCPD can be loaded to. If it's non-continuous you can load the OCPD to 100%.
        Rob

        Moderator

        All responses based on the 2017 NEC unless otherwise noted

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          #5
          Now you all are making me second guess my math...
          How would it be 2.88 per phase?
          I would have said its 2.88/3 seen per phase..

          Correct me if I'm wrong..
          If you have a 30A breaker, regardless of how many poles it is, it only protects the equipment up to the combined amperage seen across all the poles its connected to. Your 2.88kva should be you max across all three phases.
          208V*30*0.8/1.732= 2.882 kva for the given breaker
          each phase would see 960.73VA

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            #6
            WA -
            you don't want to divide by sqrt(3)
            for a balanced 3ph load:
            VA = V * I* sqrt(3)
            VA = 208 * 24* 1.732 = 8646 VA
            Without data you’re just another person with an opinion – Edwards Deming

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              #7
              WA -
              Suggest, for balanced 3ph loads:
              get rid of "amps per leg", or "amps per phase". Those terms don't mean anything.

              For example, when one says the current is 24A, this implies the current is fairly well balanced and average is 24A. An actual clamp reading might be A: 23A, B: 25A, C: 24A. yep, 24A

              If the current is not balanced, Then list the currents, for example: A - 15A, B - 25A, C - 10A, N - 9A.

              If you want to calculate the load single phase do that. You will end up with a VA by phase. Then calculate each phase amperage.
              IF you want to calculate the 3ph load, do that. You end up with a 3ph VA. Calculate the 3ph current.

              Note for later discussion: 208V single phase loads can get a bit tricky to account for the VA by phase
              Without data you’re just another person with an opinion – Edwards Deming

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                #8
                If the loads can be derated in some fashion (based on demand) from the Article 220 calculation, then the VA total from the panel schedule may exceed the panel rating. It is the demand that can't exceed the panel rating.
                Ron

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                  #9
                  Originally posted by infinity View Post
                  The 80% rule is the amount of continuous load that the OCPD can be loaded to. If it's non-continuous you can load the OCPD to 100%.
                  My understanding is that if your calculation includes 125% of all continuous load on the circuit + 100% of all non-continuous load on that same circuit, as long as your final load doesn't exceed the rating of the OCPD, you comply with the requirements. Now if the only load on the circuit is continuous, then, yes, you are only allowed to use 80% of the OCPD rating, but there is nothing that states that you can't load an OCPD up to only 80% of it's rating, correct?

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                    #10
                    Originally posted by vw55 View Post
                    . . . but there is nothing that states that you can't load an OCPD up to only 80% of it's rating, correct?
                    I don't think you asked that in the way you intended. But the answer is that you can certainly load an OCPD to 100% of its rating, if all of that load is non-continuous.


                    Charles E. Beck, P.E., Seattle
                    Comments based on 2017 NEC unless otherwise noted.

                    Comment


                      #11
                      Would this be a good time to mention that not one of the previous responses has correctly answered the original question?
                      Originally posted by Nardie View Post
                      So what should the VA load be on the panel schedule for each phase?
                      The correct answer is that each phase should be assigned one third of the total VA load of whatever components are powered by that circuit. If, for example, the circuit supplied a 150 gallon water heater for which the manufacturer called for a 30 amp circuit, and that drew 21 amps at full heating capacity, then the panel schedule would be assigned 2520 VA on each phase.


                      Charles E. Beck, P.E., Seattle
                      Comments based on 2017 NEC unless otherwise noted.

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